It is a fat-soluble vitamin that is stable to air and moisture but decomposes in sunlight. It is found naturally in a wide variety of green plants, particularly leaves, since it functions as an electron acceptor during photosynthesis, forming part of the electron transport chain of Photosystem

Vitamin K is a group of structurally similar, fat-soluble vitamins that are needed for the posttranslational modification of certain proteins required for blood coagulation and in metabolic pathways in bone and other tissue. They are 2-methyl-1,4-naphthoquinone (3-) derivatives. This group of vitamins includes two natural vitamers: vitamin K₁ and vitamin K₂.

Vitamin K₁ is also known as vitamin K_j, phylloquinone or phytomenadione (also called phytonadione). Vitamin K₁ is required for blood coagulation and is synthesized by plants, is found in green leafy vegetables, and can be found in soybean oil.

Vitamin K₂ is involved in bone metabolism. Vitamin K₂ homologs (menaquinones) are characterized by the number of isoprenoid residues comprising the side chain. Menaquinones are abbreviated MK-n, where n represents the number of isoprenoid side chains. Thus, menaquinone-4 abbreviated MK-4, has 4 isoprene residues in the side chain. Bacteria can produce a range of vitamin K₂ forms, including the conversion of K₁ to K₂ (MK-7) by bacteria in the small intestines. No known toxicity exists for vitamins K₁ and K₂.

Three synthetic types of vitamin K are known: vitamins K₃, K₄, and K₅. Although the natural K₁ and K₂ forms are nontoxic, the synthetic form K₃ (menadione) has shown toxicity.